Lower density ethylene interpolymers with moderate molecular weights will be referred herein as plastomers. Plastomers have been made with catalyst systems based on Ziegler-Natta vanadium catalysts using aluminum alkyl based activators. Such polymers have high levels of regio-inversion for the insertion of the propylene comonomer. The low activity of the catalyst leads to the need to de-ash the polymer to remove vanadium residues, especially for if the polymer is to be used for electrical applications. Propylene and 1-butene have been used as comonomers to provide short chain branching (SCB) in Ziegler-Natta produced plastomers. More recently metallocene based single site catalysts have been identified to make plastomers in a continuous solution process at higher temperatures and at higher activities where activity is defined as the amount of polymer produced per amount of transition metal single site catalyst component.
It has been recognized that metallocene based catalyst systems produce terminal unsaturation by, for example, beta-hydride elimination. It has been recognized that for many metallocene based processes the unsaturated chain end may incorporate in a growing chain and form long chain branches (LCB). EP495099 (Mitsui) and EP608369 (Dow) describe processes proving indications of LCB formation through increased shear sensitivity of the polymer, a recognized effect provided by LCB.
EP495099 uses a hafnocene in conjunction with alumoxane as an activator. In the examples 1-octene, 1-butene and propylene were used as a comonomer for batch polymerization at temperatures less than 100° C. providing polymers with high MIR values. Extensive depletion of comonomer in batch type reactions may favor LCB formation. The polymerization conditions however lead to significant levels of catalyst residues. The polymer produced at the end of the batch polymerization process may have a significantly different comonomer content than that produced early in the process and the polymer may have a broader compositional distribution.
In WO9941294/19Aug1999 and WO9945041/10Sep99 (issued as U.S. Pat. No. 6,291,609) processes using metallocene based single site catalysts are described in which the polymerization temperature is increased to above 100° C. by using hafnium as a transition metal and/or improved non-coordinating anionic activators (NCA). WO99/1294 does not combine the use of the improved catalyst system with propylene with a sufficiently low amount of diene. Polymerization is performed at 115° C. with a relatively low amount of propylene. There is no quantification of the LCB level. WO9945041 teaches the use of a bridged hafnocene activated by a tetra-aryl substituted anion, wherein each anion has at least two cyclic aromatic rings. In the examples octene-1 is used as a comonomer. Polymerization was at 170° C. for MI levels of from 0.49 to 3.6 g/10 min. WO9945041 does not teach the use of propylene as a comonomer at especially high temperature polymerization temperatures.
WO0234795 describes a plant, in which these processes can be practiced industrially and in which an advantageous arrangement is described for recycling unreacted monomers back to the polymerization reactor(s) after suitable purification through liquid phase separation, distillation and/or sieves. The above references are incorporated by reference for US purposes.
WO0037514 describes a gel-free, branched semi-crystalline ethylene propylene copolymer containing high levels of propylene. The polymerization temperature was less than 100° C. In the examples a bridged zirconocene is used with an NCA in a continuous polymerization reactor.